Brazing sheet commonly includes a core alloy bonded to a silicon-containing brazing alloy. External corrosion resistance is a concern common to many brazed aluminum heat exchangers. For example, most brazed aluminum plate type evaporators have a coating applied to the brazed assemblies to aid in corrosion protection. Commonly this is a hexavalent chromate based coating. These coatings are recognized as the industry standard from a corrosion resistance standpoint but hexavalent chromium is a carcinogen and many countries are banning its use in the near future. Hence the necessity for a highly corrosion resistant base aluminum material is now greater than ever.
The use of an interlayer as a means of alleviating intergranular corrosion problems from penetration of Si into the core alloy of brazing sheet and minimizing localized melting of the core alloy is well documented. U.S. Pat. No. 2,821,014 to Miller describes use of an interliner to avoid in very substantial measure any penetration and resultant weakening of a core alloy by a brazing filler metal. Retention of the core alloy after brazing is generally recognized as an important consideration in the determination of post-brazed corrosion resistance. U.S. Pat. No. 4,586,964 to Finnegan et al. describes a procedure including a full anneal followed by cold working of a 3xxx series core alloy (i.e., an —H1X temper) to improve post brazed corrosion resistance. The introduction of cold working after a full anneal can result in recrystallization of the core alloy which itself provides greater general resistance to Si penetration and localized erosion during the braze cycle.
The above approaches recognize that Si diffusion into the core can have deleterious effects on corrosion resistance. Neither of the approaches, by themselves, identify highly corrosion resistant, long-life products.
An approach to achieving substantially improved corrosion resistance is documented in U.S. Pat. Nos. 5,037,707 and 5,041,343, both to Fortin et al. These patents describe the use of a low Si containing (less than 0.15 wt. %) 3xxx series core alloy, fabricated to final gauge without benefit of a substantial homogenization or interannealing practice, bonded directly to a 4xxx series braze cladding containing 1–15 wt. % Si. A manganese bearing dispersoid band is described as developing within the core at a core/cladding interfacial region after the brazing cycle due to the localized diffusion of Si from the 4xxx braze cladding. The Si reduces the local solubility of Mn and precipitation of the Mn—Si dispersoids (e.g., Al12(Fe,Mn)3Si dispersoids) results in the interfacial region of Si diffusion. These Si containing dispersoids are resistant to reversion during the brazing cycle. The interfacial region becomes depleted in Mn in solid solution relative to the underlying core alloy. Corrosion attack is described as occurring preferentially within the band of precipitates before the main alloy body is attacked. Example 3 of these patents demonstrates that once the main body is attacked, corrosion occurs quite rapidly through the 3xxx core, perforating in less than 48 hours. The processes for fabricating products that are back annealed (referred to in the industry as —H2X type tempers) and fully annealed (referred in the industry as —O tempers) with corresponding annealing temperatures are also outlined.
Alloys relying on the precipitation of dense Mn bearing (e.g., Al12(Fe,Mn)3Si) dispersoids for extended corrosion resistance have found broad commercial applications for products having minimal formability requirements (i.e., in —HXX tempers), for example in radiator and heater tube applications. However, the practice described in U.S. Pat. No. 5,041,343 has not found commercial acceptance for fully annealed tempers as these alloys are susceptible to localized erosion of the core alloy when subjected to levels of cold working insufficient to result in recrystallization of the core prior to melting of the braze cladding. Fully annealed O-tempers are commonly specified for applications requiring significant formability and hence the material will be subjected to widely varying degrees of cold work during the forming operation. As a result of this localized melting (also termed “erosion”) of the core, the formation of a dense dispersoid band in the core alloy adjacent to the cladding is largely compromised. Furthermore, the braze cladding flow is poor as a result of the enrichment of aluminum from the core alloy into the braze cladding. The net result is poor brazeability and poor corrosion behavior. The problems with localized erosion in fully annealed tempers in these alloys (i.e., alloys where the core alloy does not receive a homogenization and is bonded directly to a 4xxx braze cladding) is well documented in the literature.
As a result of the problems associated with localized erosion and its compromising effects on the development of a consistent and continuous dispersoid band, the 3xxx core alloy of O-temper brazing sheet products almost universally receives a homogenization treatment. This homogenization treatment coarsens the size of the average Mn bearing dispersoid and influences the number and size distribution of the Mn bearing dispersoids in the core alloy with the net result of promoting the ease of recrystallization and/or recovery of the core during the brazing cycle. After homogenization, there are fewer small Mn particles that can revert during the braze cycle, significantly lowering the Mn levels in solid solution. This helps to alleviate localized erosion in formed parts but largely mitigates the development of a dense and continuous dispersoid band as an effective means of corrosion protection.
Hence there exists a need for an alloy and process to produce an alloy that is supplied in a fully annealed temper, can be subjected to a broad spectrum of forming strains, can be exposed to a brazing event and subsequently develops a continuous, dense dispersoid band with minimal erosion of the core alloy. Furthermore there exists a need for an alloy that retains a high inherent corrosion resistance even after the dispersoid band region corrodes away. There also is a need for products produced from O-temper brazing sheet to have exceptional corrosion resistance particularly for use in non-chromate coated brazed heat exchangers.